The present invention relates to an instantaneous voltage drop detector, and more particularly to a detector for detecting an instantaneous voltage drop in a battery used for a mobile radiotelephone and the like.
A mobile radiotelephone may experience a voltage drop which occurs in a battery supplying power during a telephone conversation, whereby its transmitter/receiver section may be controlled during on the period of voltage drop. More specifically, if the battery voltage is restored to a normal level within a predetermined period of time, for example, three seconds from the voltage drop, the transmitter/receiver section is not controlled and the telephone conversation is maintained. On the other hand, if the normal voltage level is restored more than three seconds after the voltage drop, the telephone is automatically returned to the waiting state. Unless the normal voltage level is restored even after the lapse of three seconds from the voltage drop, the mobile radiotelephone stays in an inoperable state.
In a conventional instantaneous voltage drop detector used for the mobile radiotelephone, the battery output is supplied to a reset circuit and to a regulator, respectively wherein the output is connected to an RC time constant circuit. During a telephone conversation, the outputs from the reset circuit and the RC time constant circuit are used for logically judging whether the radiotelephone should maintain the conversation or should shift to the waiting state. The RC time constant circuit provides a time, e.g., three seconds to make this judgment.
However, the conventional instantaneous voltage drop detector is inconvenient in that a detection delay occurs while the regulator output is being used for making the above-mentioned judgment. Moreover, when the battery output voltage is lowered to the median voltage level, between the uppermost and lowermost levels, the detector often cannot detect such a drop due to the effect of the RC time constant circuit connected to the regulator output. Therefore, the conventional detector cannot correctly detect the instantaneous voltage drop.